Absolute Dating
How do we determine the age of
Rocks?
Relative
Dating
RadioActive
Dating
Age of
Rocks
Law of
Superposition
Absolute
Dating
Ordering
of rock
layers
Index
Fossils
Absolute Dating
• The actual age of a rock or
fossil
• Absolute dating occurs
through the process of
radioactive dating
Radioactive Dating or Age
Radioactive dating is a way
to measure the age of a
material by comparing the
amount of decayed
radioactive element present
in the item
How do scientist tell the actual age of
rocks?
• The universe is full of naturally
occurring radioactive elements.
• Radioactive atoms are
inherently unstable; meaning
over time, radioactive “parent
atoms” decay into stable
“daughter atoms.”
How do scientist tell the actual age of
rocks?
• When molten rock cools, forming
an igneous rock, K40 (potassium
radioactive atoms) are trapped
inside.
• Once formed K40 begins to decay
at a predictable rate, and changing
into Ar40 (Aragon).
• K40 is parent atom and Ar40 is
daughter cell.
How do scientist tell the actual age of
rocks?
• Scientist will measure the quantity
of unstable atoms(K40) left in a
rock and compare it to stable
atoms(Ar40) and be able to
determine the age of that sample.
• Carbon is used to date once living
organisms.
• http://www.youtube.com/watch?v
=vYvUSro7YUM
Half-life
• The amount of time required for
half of the radioactive material to
decay in an item or in our case
the rock.
Half-life
• Rock 1st forms 100% of K40
• 1 half-life 50% of K40, 50% of Ar40
• 2 half-life 25% of K40, 75% of Ar40
• 3 half-life 12.5% of K40, 87.5 of Ar40
• 4 half-life 6.25% of K40, 93.75 of Ar40
• 5 half-life 3.125% of K40, 96.875 of Ar40
Half-life Chart
Half-Life Questions
1. If you have 10g of K40 and 2
half-life’s have passed how
many grams of K40 do you
have left?
10g of K40
1 half-life; 5g of K40
2 half-life; 2.5g of K40
Half-life Questions
1. If 50% of the K40 is gone, how old is the
rock?
• 1.3 million years old
2. If this rock is 3.2 million years old how
much K40 is still present in this rock?
• About 10%
3. How much Ar40 is present if the rock is
3.2 million years old?
• About 90%